The PHP Group released PHP 5.3.10 on Thursday in order to address a critical security flaw that can be exploited to execute arbitrary code on servers running an older version of the Web development platform.

The vulnerability is identified as CVE-2012-0830 and was discovered by Stefan Esser, an independent security consultant and creator of the popular Suhosin security extension for PHP.

SecurityFocus classifies the issue as a design error because it was accidentally introduced while fixing a separate denial-of-service (DoS) vulnerability in early January.

That vulnerability is known as CVE-2011-4885 and was disclosed in December 2011 at the Chaos Communication Congress by security researchers Alexander Klink and Julian WA$?lde.

It affects a number of Web development platforms including PHP, ASP.NET, Java and Python and can be exploited in a so-called hash collision attack. The PHP development team addressed CVE-2011-4885 in PHP 5.3.9, which was released on Jan. 10.

"The fix for the Hash Collision DoS introduced a new directive (max_input_vars) to limit the number of accepted input variables," said Carsten Eiram, chief security specialist at vulnerability research firm Secunia.

"However, due to a logic error in the "php_register_variable_ex()" function in php_variables.c certain cases are not handled correctly when the number of supplied variables is greater than the imposed limit," he explained.

This error can be exploited by attackers to remotely execute arbitrary code on a system that runs a vulnerable PHP installation. PHP 5.3.9 along with any older versions for which the hash collision DoS patch was backported, are affected, Eiram said.

Proof-of-concept code that exploits this vulnerability has already been published online, so the likelihood of attacks targeting CVE-2012-0830 are high. Web servers administrators are advised to upgrade to PHP 5.3.10 immediately.